Schools are facing the prospect of further strikes after a teaching union warned it will stage more walkouts if progress is not made in a bitter dispute over pay, pensions and working conditions.

Thousands of schools across England and Wales were forced to close their doors to some or all pupils today as members of the National Union of Teachers (NUT) joined rallies, marches and picket lines as part of an ongoing campaign of industrial action.

The NUT said the action had been a "clear demonstration" that teachers were tired of the pressure the Government is putting them under.

But the Department for Education (DfE), which has condemned the walkout, said that there was a low level of support for the strike with around one in eight schools in England forced to fully close.

NUT general secretary Christine Blower warned that more strikes could be called if there is no movement in talks between the Government and unions.

" Today has been a clear demonstration that teachers are thoroughly tired of the intolerable pressures they are being put under by the Coalition Government," she said.

"Despite being the only teachers' union to be taking action, members still felt it was essential they made a stand."

She added: " As a matter of urgency the Government needs to address the real concerns of teachers by engaging seriously in the talks with the NUT and other teacher unions. The talks should not be about implementation of Coalition policies. Issues of excessive workload, performance related pay and unfair pension changes need to be discussed and we need to move forward constructively."

Ms Blower said: "Teachers love teaching but are crushed by the long hours and stifling accountability regime. If there isn't movement in the talks there could well be further strike action this summer - perhaps David Laws can persuade Michael Gove to avoid that."

The DfE said that 12% of schools in England had been forced to fully close. This figure is based on calls to schools and local authorities, with a 90% response rate.

A spokesman said: "The NUT has tried to create as much disruption for pupils and parents today as possible. In spite of this, thanks to many hard working teachers and heads, only 12% of schools across the country were closed today. This is by far the lowest level of support for any national teachers' strike since 2010.

"Parents will struggle to understand why the NUT is striking over the Government's measures to let heads pay good teachers more. They called for talks to avoid industrial action, we agreed to their request, and talks have been taking place weekly. Despite this - and without the support of any of the other six unions engaged in the talks - the NUT has today taken industrial action. While the impact in many schools has been negligible, it has disrupted parents' lives, held back children's education and damaged the reputation of the profession."

Overall closure numbers are not yet known, but f igures for a small sample of authority suggest that the industrial action has affected at least some schools in every area.

Around 50 schools in Cumbria are thought to have at least partially closed, along with around 64 in Hampshire, about 32 in Medway, and about 75 in East Sussex, figures on council websites suggest.

In Norfolk around 63 schools appear to have been affected, as well as about 53 in Warwickshire, about 58 in Coventry, 67 in Cornwall, 24 in Lincolnshire and 97 in Cardiff.

Many secondary schools, and a growing number of primaries, in England are now academies and not under local council control, so authorities may not know whether they have been affected by the walkout.

As NUT members began their walkout, there were indications that relations between the NUT and fellow teaching union the NASUWT - which together have been running a joint campaign of industrial action - have become strained.

Both unions took part in a series of regional strikes in the autumn term, but the NASUWT decided not to take part in today's national action.

A leaked memo signed by NASUWT general secretary Chris Keates claims that some members have faced threats, insults and intimidation from members of the NUT.

The document, sent to senior local NASUWT officials, which has been posted on social networking websites, says: " We should not tolerate any threats, insults or attempts to intimidate our members or activists by the NUT. Unfortunately, in some areas, this has been a hallmark of the activity to date. The NASUWT, as an independent trade union, has made its decision with regard to industrial action strategy and that should be respected by a sister trade union."

The memo does also say that the NASUWT remains " committed to the joint declaration and seeking to work with the NUT".

An NUT spokeswoman insisted that the union had not been running a national campaign against the NASUWT.

"There has been no national campaign against NASUWT members regarding strike action," she said.

"There has been no negative campaigning from the national NUT headquarters and the NASUWT has not brought this to our attention. We continues to engage in talks with Government alongside the NASUWT and other teacher unions to resolve the very pressing issues that face the teaching profession."

An NASUWT spokesman declined to comment on the memo or the NUT's strike action.

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